MCQ – 89945

Cyanosis is apparent when the mean capillary concentration of reduced hemoglobin exceeds

A.	2 gram / dL
B.	3 gram / dL
C.	4 gram / dL
D.	5 gram / dL

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Correct Answer » D Explanation

Detailed Explanation:

– Clinical Reasoning for the Correct Answer:
– Cyanosis (bluish discoloration of skin/mucous membranes) is visible when reduced (deoxygenated) hemoglobin in capillary blood exceeds a threshold.
– The classic teaching: cyanosis becomes clinically detectable when the concentration of reduced hemoglobin rises above 5 g/dL in capillary blood.

– Why Option A (2 g/dL) is Incorrect:
– 2 g/dL is too low; not sufficient for visible cyanosis.
– Such a level may exist in mild hypoxia—skin appears normal.

– Why Option B (3 g/dL) is Incorrect:
– Cyanosis is generally not seen until levels exceed 5 g/dL.
– 3 g/dL is below the threshold for clinical detection.

– Why Option C (4 g/dL) is Incorrect:
– Although approaching the threshold, cyanosis is still rarely detectable at 4 g/dL.
– Significant clinical studies and textbooks agree detection is at 5 g/dL.

– Why Option D (5 g/dL) is Correct:
– 5 g/dL is the widely accepted threshold for visible cyanosis.
– This value holds regardless of total hemoglobin (i.e., polycythemia may exhibit cyanosis at high oxygen levels and anemia may not show cyanosis despite severe hypoxemia).

Differential Diagnoses
– Central cyanosis: low arterial oxygen saturation (cardiac, pulmonary, methemoglobinemia).
– Peripheral cyanosis: sluggish blood flow (congestive heart failure, local vasoconstriction).
– Pallor (severe anemia): no cyanosis due to low total hemoglobin.